The present invention relates to methods of reproducing and using by-products formed in a production process of lactic acid based compositions. As the lactic acid based by-products, for example, there are a lactic acid component contained in a condensed liquid, residue and waste liquid from a vacuum pump in a process for synthesizing lactic acid prepolymer or polylactic acid; a residue and waste liquid from a vacuum pump in a process for depolymerizing lactic acid prepolymer to obtain lactide; and waste polymer formed in a process for obtaining polylactic acid by polymerizing a waster mother liquid, waste washing liquid, residue and lactide in a process for refining lactide. Lactic acid components reproduced according to the method of the present invention are used for production of lactic acid, lactide or polylactic acid having a desired optical purity.
Polylactic acid has high safety for organisms, and lactic acid as a decomposed substance thereof is absorbed in vivo. Since polylactic acid is a high molecular compound having high safety for organisms, it has been used for medical purposes, such as a stitching thread for an operation; drug delivery, i.e. time release capsule; and a reinforcing material for fracture of a bone. Since polylactic acid can be decomposed under a natural circumstance, it is also receiving attention as decomposable plastics. Further, polylactic acid has been used for various purposes, such as a uniaxially or biaxially oriented film, fibers, injection molding products or the like.
As methods for producing polylactic acid, there are a direct method, wherein lactic acid is directly dehydrated and condensed to obtain an objective polylactic acid; and a method, wherein cyclic lactide, i.e. dimer, is once synthesized from lactic acid, and then subjected to crystallization or fractionation for refining thereof and then ring-opening polymerization. Methods for synthesizing, refining and polymerizing lactide are disclosed in U.S. Pat. No. 4,057,537; EP Patent Application Publication No. 261,572; chemical literatures, such as Polymer Bulletin, 14, 491-495 (1985); and Makromol. Chem., 187, 1611-1628 (1986). Also, Japanese Patent Publication (KOKOKU) No. 56-14688 discloses that a dyad cyclic diester as an intermediate is polymerized in the presence of tin octylate as a catalyst to produce polylactic acid.
However, in case lactide or polylactic acid is produced by the conventional method, by-products and unnecessary substances as described hereinunder are inevitably produced:
a. a lactic acid component contained in a condensed liquid, waste solvent, residue and waste liquid from a vacuum pump formed in a process where lactic acid prepolymer, polylactic acid or lactide is directly synthesized from lactic acid; PA1 b. a residue and waste liquid from a vacuum pump formed in a process where lactic acid prepolymer is depolymerized to obtain lactide; PA1 c. a waste mother liquid, waste washing liquid and residue formed in a process where lactide is refined; PA1 d. loss in a process where lactide is polymerized to obtain polylactic acid; and PA1 e. loss in a process where polylactic acid is subjected to a molding operation.
Also, in case the above mentioned wastes are abandoned, an expense for abandonment increases a cost of the product. Thus, reduction of in the wastes would have great industrial advantages.
Therefore, if the above stated by-products and waste polylactic acid on the market are reused, not only a yield as a whole is increased but also the cost thereof is reduced.
However, when these by-products and waste polylactic acid are reused, there are following problems.
1. In the above a, b and c, the wastes have different optical purities according to processes for obtaining objective products. Also, even if the wastes are formed from the same process, optical purities thereof vary. Therefore, in case these wastes are directly reused, it is difficult to obtain lactic acid, lactide or polylactic acid having a stable optical purity.
2. In case the above d, e or waste polylactic acid is used, a method for obtaining lactide by depolymerization or a method for obtaining lactic acid by hydrolysis may be employed. However, since it is difficult to carry out the depolymerization or hydrolysis under a mild condition, racemization takes place in a process of the depolymerization or hydrolysis, so that lactic acid, lactide or polylactic acid having a stable optical purity can not be obtained.
On the one hand, physical characteristics, such as crystallinity and drawing orientation, of the polylactic acid greatly fluctuate according to the optical purities thereof. Therefore, it is necessary to control the optical purities of polylactic acids to a fixed degree in general use.
Also, optical purities of polylactic acids required on the market vary from 0% e.e. to 100% e.e. Especially, polylactic acids having optical purities of 85% e.e. to 100% e.e. are most required in general use.
The following methods for obtaining polylactic acids having various optical purities are known:
(1) a racemization reaction is controlled by adjusting reaction conditions, i.e. heat history, in a process for synthesizing lactide to obtain lactide having a desired optical purity.
(2) a racemization reaction is controlled by controlling reaction conditions, i.e. heat history, in a process for directly synthesizing polylactic acid from lactic acid without obtaining lactide to obtain polylactic acid having a desired optical purity.
(3) lactide having a high optical purity and lactide having a low optical purity are mixed at a predetermined ratio so that polylactic acid having a desired optical purity can be obtained.
However, methods (1) and (2) require delicate control of the processes, so that products having a stable optical purity can not be obtained. In method (3), DL-lactic acid and DD-lactic acid used for obtaining lactides having a low optical purity are more expensive in cost than LL-lactic acid, so that a cost for obtaining polylactic acid is also increased.
Therefore, an object of the invention is to provide a method for obtaining lactic acid through reproduction of by-products formed in a process for producing lactic acid based compositions.
Another object of the invention is to provide a method for obtaining polylactic acid having a desired optical purity by using the obtained lactic acid.
A further object of the invention is to provide a method for obtaining polylactic acid having a desired optical purity as stated above, wherein polylactic acid having a stable optical purity can be easily obtained at a low cost.
Further objects and advantages of the invention will be apparent from the following description of the invention.